Laser soldering is widely used in the electronics industry when the assembly of printed circuits, electronic components, conductors and microelectronic devices requires the formation of a permanent joint between two or more metal parts or metallic surfaces. Solder as used herein is defined as a metal or metallic alloy which is melted to join two or more metallic parts or surfaces. The term "electronic device" will be used to refer to products produced by the electronics industry.
Prior art laser soldering processes typically involve the application of a solder paste or solder preform to at least one part having a metallic surface during the manufacturing process. If solder paste is used, sufficient heat is then applied to reflow or melt the solder paste. Ideally, this molten solder wets the metallic surfaces to be joined. The molten solder is then cooled to provide a solder preform or pad. The second part having a metallic surface is then positioned so as to be in contact with the solder. The steps of applying solder and reflowing it may be done as part of the manufacture of the electronic device or may be done separately. In the latter case, a solder preform or pad results which is supplied in a ready to use fashion. It is not necessary to reflow the solder preform. The second metal part is simply positioned thereon.
Thermal energy is directed at the junction of the two metallic parts and the solder pad or preform. A permanent joint results from the alteration of the structure and composition of the metallic surfaces and solder caused by the extremely high temperatures and intensity of the laser energy.
However, the presence of metal oxides, carbon compounds and other contaminants on the surface of the metal parts or metallized surfaces significantly impedes the formation of a metallurgically sound solder joint. Such contaminated metal surfaces cannot be wet by the molten solder. Metal oxides, whether on the metal surfaces or in the solder paste, are particularly detrimental to the production of solder joints with high metallurgical integrity. The quality of the solder joint is particularly important in the electronics industry where the joint must have sufficient integrity to hold the various electronic components in place and to pass electrical signals. Without metallurgically sound solder joints, the overall performance and reliability of electronic devices may be comprised by low quality or imperfect joints.
Traditionally, fluxes have been used to remove the metal oxides and metal oxide films from the metallic surfaces which must be wet by the molten solder. The formation of sound metallurgical joints have generally been obtainable only with the use of a flux capable of removing metal oxides from the metallic surfaces and inhibiting any subsequent oxidation of the cleaned metal surface. Additional benefits achieved with traditional fluxes included the removal of metal oxides in the molten solder and a resultant reduction in the surface tension of the molten solder and an enhanced flow. Also such agents have generally assisted in the transfer of heat to the joint during the actual soldering process.
Traditional fluxes commonly consist of an active agent dissolved or dispensed in a liquid carrier which evaporates in the soldering process. The function of the active agent is to reduce base metal oxides. Active agents may be organic or inorganic acids. Well known prior art fluxes are natural rosin based fluxes such as wood rosin or gum rosin, the major component of which is abietic acid. A typical prior art flux contains about 80% rosin.
Prior art laser soldering processes employ a variety of techniques to apply fluxes. They are generally brushed or sprayed when used in the manufacture of electronic assemblies. It is sometimes desirable for the flux to have a low viscosity so that it can readily flow into gaps of the electronic assemblies to encompass the entire surface. Alternatively, conventional rosin based fluxes may be incorporated within the center or core of solder wires.
However, several significant problems arise in the manufacture of electronic devices as a result of the use of traditional fluxes. After a solder joint is formed, a flux residue remains. Such residues generally consist of unreacted flux, carrier which is not evaporated, acid or salt deposits, and/or the removed metal oxides. These residues can be deleterious to the long term reliability of the electronic device if not removed. Unreacted flux or non-volatilized carrier can absorb water to become an ionic conductor which can result in electrical shorting and corrosion. The active portion of the unreacted flux can, over a period of time, corrode the soldered components and cause electrical faults. Furthermore, in traditional prior art soldering processes, the flux residue covers the entire assembly and is present not only on the surface, but also underneath specific components where inspection is difficult and the residue is hard to remove.
Steps to remove the unreacted flux and flux residue typically require the use of solvents in which the unreacted flux and residue are soluble. Typical cleaning solvents are the halogenated hydrocarbons such as chlorinated fluorocarbons. The use of such halogenated solvents is particularly non-desirable because of their deleterious environmental effects.
In addition to these problems, the post-soldering presence of unreacted flux and flux residues can cause significant problems with respect to subsequent manufacturing steps and the performance of other components in the electronic device.
For example, in many electronic and microelectronic devices, especially thick film products, a silicon based potting material or gel is applied to the device to protect the electrical circuitry from the negative effects of moisture. Potting is typically the last step in the manufacturing process. The presence of the flux material can inhibit the cure of the potting material and thus render the circuitry vulnerable to attack from water and moisture in the atmosphere. In particular, the partially cured potting material allows moisture to condense at the circuit board/gel interface. When the device is powered during operation, the condensed moisture induces high leakage current which can (1) interfere with the current operation and (2) cause failure through dendritic growth.
Finally, the application of heat to the flux can result in the volatilization of particular flux components which can have unpleasant odors and non-desirable environmental effects.
The problems attendant to the use of flux agents are exacerbated in laser soldering processes. By its very nature, laser soldering is incapable of providing a heat profile whereby the temperature of the solder joint is allowed to have a gradual or ramp increase and a corresponding gradual or ramp decrease. The heat profile for a laser soldered joint appears merely as a sharp spike, exhibiting neither a ramp increase or ramp decrease in temperature. As a result, even liquid fluxes known as "no clean fluxes" are unsuitable for use in laser soldering processes. Such no-clean fluxes depend on a heat profile produced during conventional soldering to volatilize or decompose most of the flux constituents so that little or no flux residues result.
Another problem associated with laser soldering is the deflection of some or all of the laser beam energy away from the joint to be soldered. This deflection is generally attributed to the natural reflectance of the metal or metallic parts. Although seen to some extent with all laser sources, it is particularly severe when the source generates long wavelengths.
Finally, the heat energy generated by the laser beam may not be conducted to all of the components of the desired solder joint. Alternatively, insufficient heat may conducted. Although the laser beam will generally only hit the top surface of the upper most part to be joined, sufficient heat must be conducted to the bottom metal part and the solder to effect the necessary structural changes therein. This is true regardless of whether the solder is in the form of a preform pad or is molten. If a pad is utilized, enough heat to reflow or melt the pad must also be supplied.
Prior art patents such as U.S. Pat. Nos. 4,092,182 and RE 30,696 to Arbib et al., disclose the use of a flux composition having at least one ester derived from a polyhydric alcohol and at least one saturated or unsaturated fatty acid or monocarboxylic mononuclear aromatic acid. The ester has a molecular weight of at least 300. The composition also has at least one additional constituent selected from (1) organic acids which are substantially soluble in the ester when in a molten condition, (2) flux activating agents, and (3) flux residue hardening agents. The ester is at least 25% by weight based on the total weight of the ester and the additional constituent. However, the laser soldering processes disclosed in the Arbib et al. patents result in the presence of the composition on other components of the electronic devices, such as resistors, thereby impeding and in some cases completely precluding effective laser trimming.
Those skilled in the art will appreciate that in the manufacture of thick film electronic devices, resistors are produced by printing resistor ink in the desired locations and then firing to cure the ink. Due to process variations, such resistors are almost always out of specified range or value. As a result, the resistors must be cut or trimmed by a laser beam to correct their value. Trimming cuts away excess cured resistor ink to produce the desired performance values. The trimming laser "sees" the contrast between the black resistor and the surrounding white border. The presence of the Arbib et al. compositions can impede machine vision, i.e. the "seeing" of the laser and can actually prevent the cutting away of the resistor ink because of their energy absorptive capabilities.
Although some resistors can be trimmed prior to the laser soldering process, others are used to compensate for any process variations. These resistors can only be trimmed as part of the final `end of line` functional test. The end of line test serves as the final quality control check and ensures that the device operates as intended.
Accordingly, what is desired is a soldering process which eliminates the use of traditional fluxes and does not inhibit the cure or performance of silicon-based potting compounds or the trimming of the finished electronic device. The process must, however, produce solder joints which are metallurgically sound and which are without corrosive residues that would require post-solder cleaning.
It is thus an object of the invention to provide a process of laser soldering for use in manufacturing electronic devices which utilizes an energy absorptive coating in such a manner that the need for traditional fluxes is eliminated. The desired process should not impede subsequent manufacturing steps such as the use of silicon-based potting compounds and the final trimming of the electronic device.
It is a further object of the invention to provide a process of laser soldering which utilizes an energy absorptive coating upon at least one of two metal parts to be joined. The coating should protect against metal oxide formation on the surface of the first metal part and decrease the reflectivity of the first metal part while producing a solder joint which is free of residues requiring post solder cleaning.